1. Field of the Invention
The present invention relates to a method of processing substances to be decomposed and also to an apparatus to be used for such a method, and more particularly to a method of decomposing substances to be decomposed and an apparatus to be used for such a method.
2. Related Background Art
With developing industries, a large amount of organochlorine compounds, such as chlorinated ethylene and chlorinated methano, have been used, and the disposal of such compounds has become a serious problem. In particular, gases of spent compounds are causing great concern over environmental pollution. Extensive efforts have been made to solve this problem.
For example, one of the methods that have been proposed to treat these compounds includes adsorbing contaminants such as organochlorine compounds in the natural environment on adsorbent such as activated carbon to remove the compounds from the environment. When activated carbon is used, the activated carbon that has adsorbed contaminants may be incinerated and reclaimed, or the activated carbon may be heated with steam for reclamation and the liquid containing the contaminants is recovered.
Methods for decomposing those compounds include a method for decomposing chlorinated ethylene with an oxidizing agent or catalyst. More particularly, a method for decomposing the compounds with ozone and a method for decomposing the compounds by irradiating ultraviolet rays in the presence of hydrogen peroxide are known. Also, the use of sodium hypochlorite as an oxidizing agent has been suggested. Moreover, there has been known a method for decomposing the compounds by irradiating an alkaline water suspension of photocatalytic fine particles composed of semiconductor oxide particles such as titanium oxide particles and liquid chlorinated ethylene with light.
Also, photolysis processes in which compounds are decomposed by irradiation with ultraviolet rays in a gaseous phase without using an oxidizing agent have been tried. For example, there is proposed a method in which exhaust gas containing an organohalogen compound is irradiated with ultraviolet rays thereby changing the compound into acidic decomposed gas, and then the same is washed with alkali to be made innoxious. There has also been proposed an apparatus in which drain containing organohalogen compound is aeration treated, and discharged gas generated through the aeration treatment is irradiated with ultraviolet rays and then washed with alkali. Also, a method for decomposing chlorinated ethylene with iron powder is known. In this method, it is assumed that a reducing decomposition is taking place. Also, a process for decomposing tetrachloroethylene (hereafter referred to as “PCE”) using silicon fine particles has been proposed.
It is known that chlorinated aliphatic hydrocarbons such as trichloroethylene (hereafter referred to as “TCE”) and PCE are decomposed by microorganisms aerobically or anaerobically, and there have been attempts to decompose such compounds and purify the natural environment by such a microbial process.
A variety of decomposition and purification apparatuses have been proposed to solve the problems associated with the methods and apparatuses mentioned above. For example, in one of such apparatuses, gas containing chlorine gas is mixed with organochlorine compounds in a gaseous phase to make a mixed gas, and the mixed gas is irradiated with light to decompose the organochlorine compounds.
In this example, a convenient and safe way of obtaining gas containing chlorine gas is to use chlorine gas that is generated from a chlorine-containing solution.
FIG. 1 schematically shows one example of an apparatus to generate chlorine gas. The apparatus includes a gap generation unit 11 that generate air containing chlorine gas, a water tank 12 that contains a solution containing chlorine gas, a pipe system 18 for blowing air into the solution, and a valve 14 for adjusting the amount of air. Air that passes through the aqueous liquid containing chlorine becomes to be air that contains chlorine gas, which is then introduced into a reaction vessel 5. The apparatus also includes a unit 1 that supplies substance to be degraded. The unit 1 introduces substance to be degraded into the reaction vessel 5, where the substance is mixed with the air containing chloride gas, irradiated with light by a light irradiation unit 4 for a predetermined period of time, and decomposed. After the decomposition, generated gas is discharged through an exhaust pipe 6. A chlorine solution that is contained in the water tank 12 may be a solution characterized by a hydrogen ion concentration (pH) between 1 and 4, and a residual chlorine concentration between 5 and 150 mg/L. Such a solution can be obtained by, for example, dissolving hypochlorite (e.g., sodium hypochlorite and potassium hypochlorite) in water. When an inorganic acid is contained in this solution, chlorine gas can be more effectively generated.
The above-described decomposition and purification apparatus for decomposing gaseous organochlorine compounds starts the decomposition reaction by mixing chlorine gas that is supplied from chlorine-containing water and gaseous organochlorine compounds that are substance to be degraded under light irradiation. However, not all of the chlorine gas is consumed by the decomposition reaction, but a major part of the chlorine gas is discharged intact. In other words, since a major part of the chlorine gas is discharged, only a small amount of chlorine among the entire chlorine is used for the decomposition reaction, and therefore the decomposition efficiency is believed to be lowered.
As described above, a number of methods for decomposing various organochlorine compounds have been proposed. However, it was found that most of these methods require complicated apparatuses for decomposition, and/or a further process to make the decomposition products harmless. Accordingly, there is still a demand for technology for decomposing organochlorine compounds with an eco-friendly, practical apparatus but without requiring special facility. In other words, there is still a strong demand for simpler and more effective methods for decomposing substances to be degraded and decomposition apparatuses to be used for such methods, as well as a strong demand for effective methods for decomposing substances to be degraded with a less drainage amount and without requiring a process using activated carbon or microorganisms or secondary environmental contamination, and apparatuses to be used for such methods.